If you are a SAP assessor, the transition to the Home Energy Model (HEM) requires practical preparation across several areas: learning a new methodology, adapting to significantly greater data requirements, understanding the ECaaS calculation platform, and adjusting your workflow for longer assessment times. This guide walks through each change in detail and provides a practical checklist for getting ready. The key message is that HEM is not simply an update to SAP — it is a fundamentally different calculation engine, and the sooner you begin preparing, the smoother the transition will be.
Timeline — When You Need to Be Ready
The transition from SAP to HEM is directly tied to the Future Homes Standard (FHS) implementation timeline. The government has proposed a dual methodology period during which both SAP 10.3 and HEM can be used to demonstrate compliance:
| Phase | Expected Period | Compliance Methodology |
|---|---|---|
| Current regulations | Until FHS comes into force (late 2026) | SAP 10.2 for Part L 2021 |
| Dual methodology period | Late 2026 – late 2027 (~12 months) | Either SAP 10.3 or HEM via ECaaS |
| Post-transition | Late 2027 onwards | HEM via ECaaS (primary route) |
During the dual methodology period, plots registered before the FHS comes into force may still commence under current Part L 2021 rules for up to 12 months. Plots registered after the FHS comes into force must comply immediately. For full details on the transitional arrangements, see our Transitional Arrangements page.
New Data Requirements
The most immediate practical impact for assessors is the step change in data requirements. HEM's half-hourly dynamic simulation requires far more granular input data than SAP's monthly calculation. Below is a detailed breakdown of the new data points you will need to collect.
Site-Specific Data
- Site altitude — affects external temperature and pressure calculations at each timestep
- Noise nuisance potential — influences assumptions about window-opening behaviour in the ventilation model
- Detailed orientation data — HEM's half-hourly solar calculation is sensitive to precise building orientation
Building Geometry
- More precise measurements than SAP requires — HEM models the thermal envelope in greater detail
- Zone definitions — whereas SAP uses just two zones (living area and rest of dwelling), HEM supports user-defined zones for more accurate modelling
- Thermal bridging details — linear and point thermal bridges are calculated at every timestep, so junction details matter more
Hot Water Outlets
- Individual outlet specifications — every tap, shower, and bath must be individually specified
- Tap flow rates — the actual flow rate for each hot water outlet
- Shower types — mixer, electric, or pumped, with specific characteristics
- Pipework lengths and insulation — detailed hot water distribution pipework, including pipe diameters, lengths, and insulation levels
Product-Specific Data
- Heating equipment: Specific make and model from the PCDB for every heat generator (heat pump, boiler, etc.)
- Ventilation equipment: Make and model of MVHR or MEV units, including heat recovery efficiency and specific fan power
- Heat emitters: Radiator or underfloor heating specifications
- PV systems: Panel specifications plus inverter specifications (make, model, efficiency)
- Controls: Specific heating control and thermostat details
The Assessment Workflow — SAP vs HEM
The assessment workflow changes significantly under HEM. The table below compares each stage of a typical assessment:
| Stage | SAP Workflow | HEM Workflow |
|---|---|---|
| 1. Data collection | Collect from drawings; generic product categories acceptable | Collect detailed specs; specific make/model required for all equipment |
| 2. Data entry | ~20 minutes for a standard house type | ~1 hour 40 minutes for a standard house type |
| 3. Calculation | Run in local SAP software; near-instantaneous result | Submit to ECaaS API; 5–10 minute calculation time |
| 4. Review results | Immediate compliance check; quick adjustments | Wait for results; each iteration adds 5–10 minutes |
| 5. Design iteration | Rapid trial-and-error (seconds per run) | Slower iteration cycle (minutes per run); plan changes carefully |
| 6. Submission | Via SAP software to Building Control | Via ECaaS to Building Control (standardised output) |
The longer turnaround for each calculation run means that assessors will need to be more deliberate about design iterations. Instead of rapid trial-and-error, you will need to plan specification changes more carefully and potentially batch multiple adjustments into single runs.
Understanding ECaaS
ECaaS (Energy Calculation as a Service) is the cloud-based platform that delivers HEM to the industry. Understanding how it works is essential for assessors.
How ECaaS Replaces Third-Party Engines
Under the current SAP regime, multiple software providers develop their own implementations of the SAP calculation. This has led to documented inconsistencies between providers — the same dwelling can produce different results depending on which software is used.
ECaaS eliminates this by providing a single, centralised calculation engine maintained by MHCLG. Software providers no longer build their own calculation engines. Instead, they build user interfaces that send data to the ECaaS API and display the results. This means:
- Every assessor gets the identical calculation, regardless of which software they use
- Updates to the HEM engine are applied centrally and take effect immediately for all users
- The calculation engine is built in Rust for performance, based on MHCLG's open-source implementation
- A basic web front-end is being developed for stakeholder familiarisation
What This Means in Practice
Your day-to-day experience will still involve using assessment software with a graphical interface — the difference is that when you press “calculate”, the data is sent to the ECaaS API rather than being processed locally. The 5–10 minute calculation time reflects the greater computational complexity of HEM's half-hourly simulation compared to SAP's monthly method.
Calculation Time
One of the most notable practical differences is calculation speed. SAP calculations are near-instantaneous because the monthly method is computationally simple. HEM's half-hourly dynamic simulation across an entire year (17,520 timesteps) is significantly more demanding:
- HEM calculation time: 5–10 minutes per run
- SAP calculation time: Near-instantaneous
This affects the iterative design process that many assessors are accustomed to. In SAP, you can quickly test multiple specifications to find the optimal compliance path. In HEM, each iteration requires a new calculation run, so you need to plan your approach more carefully. Experienced assessors recommend preparing a complete dataset before the first run, then making targeted adjustments based on the results.
Training and CPD
The government has not yet published final details on HEM qualification requirements. However, the scale of change from SAP means assessors should expect significant training needs:
- Methodology training: Understanding HEM's half-hourly dynamic simulation, how it differs from SAP, and how input changes affect results
- Data collection: Learning the expanded data requirements and developing efficient collection processes
- ECaaS platform: Familiarising yourself with the API-based workflow and the specific software interfaces your provider develops
- Building physics: A deeper understanding of building physics concepts (thermal mass, dynamic heat balance, pressure-driven ventilation) will help you interpret HEM results and advise clients
- Product knowledge: Understanding how specific product characteristics (heat pump COP curves, MVHR heat recovery efficiency, PV inverter efficiency) affect the calculation
Certification bodies including Elmhurst, ECMK, Quidos, and Stroma are expected to update their training programmes ahead of the FHS coming into force. Contact your certification body for details on upcoming HEM training courses.
Current Limitations
As of early 2026, the HEM test environment has several limitations that will be addressed before full deployment:
- Only electricity can be modelled as an energy supply — gas and other fuels are not yet available
- Only heat pumps are available for heat generation — boilers and other heating systems are not yet implemented
- Only radiators are selectable as heat emitters — underfloor heating is not yet available
- Only homes with hot water cylinders can be modelled — combi systems are not yet supported
Practical Preparation Checklist
Frequently Asked Questions
What new data do I need to collect for a HEM assessment?
HEM requires significantly more data than SAP. New data points include: site altitude and noise nuisance potential, detailed building geometry, individual hot water outlet specifications (tap flow rates, shower types), detailed pipework lengths and insulation, specific product data (make/model) for all heating and ventilation equipment from the PCDB, and inverter specifications for PV systems. Generic categories are no longer sufficient — you need the actual manufacturer and model number for every component.
How do punitive defaults work in HEM?
When data is missing, HEM applies default values that represent worst-case assumptions — deliberately more severe than SAP's reasonable mid-range defaults. For example, if you do not specify the make and model of a heat pump, HEM will assume a significantly lower COP than the product would actually achieve. Incomplete assessments produce much worse compliance results, making complete data collection essential.
Will my existing SAP software work with ECaaS?
Your existing SAP software will not directly work with ECaaS — software providers need to build new interfaces on top of the ECaaS API. However, during the transitional period (expected late 2026 to late 2027), you can continue using SAP software via the SAP 10.3 route. Most major SAP software providers are expected to develop ECaaS-compatible interfaces.
How long is the dual methodology period?
The dual methodology period is expected to last approximately 12 months, from when the FHS comes into force (expected late 2026) until the transitional period ends (expected late 2027). After that, HEM via ECaaS will be the only route for Part L compliance. See our Timeline & Status page for the latest dates.
What should I do now to prepare for HEM?
Start by familiarising yourself with HEM's technical documentation on GOV.UK. Explore the ECaaS test environment if you have access. Develop data collection templates covering all HEM-specific fields, and discuss data workflows with your regular clients. Sign up for certification body updates about HEM training, and follow the FHS consultation response for confirmed timelines.
Related Pages
FHS Compliance Pathways
HEM vs SAP 10.3 routes, the notional building approach, and how to demonstrate compliance.
Transitional Arrangements
Plot registration deadlines, commencement definitions, and strategies for the transition period.
SAP vs HEM
Side-by-side comparison of the old and new calculation methodologies.
Timeline & Status
Live tracker of FHS, HEM, and EPC reform milestones and deadlines.